It is known to make various motor-vehicle parts—e.g. tie rods, B-columns, struts, door beams—of hardened steel with uniform ductility and grain structure throughout the entire workpiece. The main factor affecting grain structure and/or ductility is the maximum temperature to which the workpiece is heated, that is whether or not it reaches any of several critical temperatures, the so-called AC1–AC4 points. The grain structure/ductility is thus adjusted by a heat treatment of the part, raising it to a predetermined temperature and then quenching it in accordance with the desired characteristics of the finished product.
For specific parts, however, it is desirable for the grain structure to vary from one region to another. One region might need to have exceptional strength while another might need to be able to deform somewhat. This can be accomplished most simply by making the part as a composite of two pieces that are differently treated to have the desired characteristics.
U.S. Pat. No. 5,972,134 describes a one-piece part having regions of different ductility. It is produced by heating it locally before deforming it into the desired shape and then cooling it. The disadvantage of this method is that it is a complex batch operation that does not lend itself to the mass production needed for motor-vehicle manufacture.
German utility model 200 14 361 published 16 Nov. 2000 describes a door post or so-called B-column that is rendered austenitic in a furnace and then is simultaneously deformed and quenched in a die. Some parts of the workpiece are insulated before it is put in the furnace so that they do not become austenitic and thus do not develop a martensitic grain structure. Such a process is also unwieldy, involving the application and removal of insulation before and after the heat treatment, two extra steps that considerably elevate the cost of the workpiece.
Complicating the problem of making a part with regions of different ductility is the necessity of coating one of the parts, in particular when a corrosion-resistant coating, like zinc or aluminum, is applied hot. When a workpiece needs to be constructed having two regions of different ductility and also needs to be corrosion coated, the problems multiply, making for a very expensive manufacturing process.
In copending application Ser. No. 10/744,773 filed 23 Dec. 2003 a hardenable steel workpiece having two regions is first hardened so that both regions are of generally the same low ductility. Then only one of the regions is hot coated so as to increase the ductility of the one region while not heating and changing the ductility of the other region. This procedure is relatively effective but fairly cumbersome.
Another consideration is that some parts, particular of motor vehicles, have different requirements as to corrosion resistance in different regions of the part. For example the lower region of a B-column often needs a very good hot-zinc corrosion protection while its upper region is often sufficiently protected by the paint or lacquer that is normally applied to all parts. Such a B-column often needs, as mentioned above, to have a fairly ductile foot region so that in an accident it can bend while its upper region needs to be very strong to support the roof in a rollover. Creating such a part that has regions of different ductility and regions that do not need corrosion coatings is particularly complex.
Applying a coating after heat-treating a workpiece is somewhat problematic since the heat of the coating process can detemper it. As mentioned above with reference to the copending application, this fact can be exploited so that in effect the ductility-decreasing heat-treatment is effected by the coating process. But this is not always the case and it is often the case that the coating must be applied to a part of low ductility and high strength. For example, one cannot anneal a hot-dipped galvanized piece of steel, as the high temperature of the heat-treatment process reliquefies the zinc and leaves the finished product at least partly uncoated. The coating often is vaporized so that not only does the heat treatment destroy it, but it stains or damages the underlying metal.